Traffic Engineering (fwd)
Sean M. Doran
smd at clock.org
Thu Sep 18 22:17:18 UTC 1997
Eric Germann <ekgermann at cctec.com> writes:
> At 04:31 PM 9/18/97 -0400, Sean M. Doran wrote:
> >Perhaps you could explain to me how you can find the
> >shortest path between A and B using ping times, traceroute
> >hop counts, and AS_PATHS observed at C, assuming that
> >traffic between A and B is not exchanged through C?
> You're not trying to find it between A and B. A connects to B, but B has
> every intention of redirecting to C1, C2, or C3, etc.
The originally proposed idea was that search engines would
present a list of hits sorted by the proximity to the
client that made the query.
What you are doing is rehashing the IBM scheme.
Unfortunately the IBM scheme requires
* an initial connection to a server
* each server must know the location of its
* each server must be able to determine the
proximity of its replicated data to the
client in reasonable time and without
placing too much additional load on the
server, the client, or the replicating sites
Point (1) means that for short transactions, preserving
locality is a net loss. If it is more work to redirect
than to serve directly, there is no point in not serving
Point (2) means that copies of the data that may be nearer
the client may not be known about. This is also a scaling
Point (3) is a difficult engineering challenge, not least
because determining proximity (assuming proximity is a
function of bandwidth, delay, and infrastructure) without
inducing traffic load is hard.
What is really needed is a scheme such that
* the client learns the identity of the datum
desired, and the ultimate, authoritative source
of the datum, through user interaction (typing
in a URL) or through a search database
* the client or its proxy asks increasingly distant
infrastructure if replicas are available; Van
Jacobson suggests this is a good application for
multicast and I'm inclined to agree somewhat
* the client or its proxy retreives the found
replicated datum if a replica exists and is
found within a reasonable time or scope of
the client or its proxy turns to the ultimate
source for a copy
* should the client or its proxy hear a query for
this same datum, it will offer it up for
retrieval to other clients
that is, everything in your client or proxy's
local cache may be served up to nearby clients
Note that the squid caching system emulates this scheme to
a large degree.
The difficulty is now reduced to constructing a spanning
tree for all multicast queries and determining whether
something found by the client or its proxy to be
topologically nearby is a good choice for serving up the
data. These problems seem much more tractable than the
ones the IBM scheme needs to solve, and a great deal of
theoretical work has gone into this sort of model already.
> Anyone know of some reasonably available methods for
> measuring end to end "performance" which are almost
> universally implemented?
No. See the CAIDA or IPPM efforts for why not.
P.S.: Curtis Villamizar had another interesting approach
which involved pushing content far afield to
machines with the same transport-layer (IP)
addresses, relying upon closest-exit routing to
connect one to the topologically-closest replication
machine. Unfortunately, while this could be really
cool for NSPs to offload stuff towards peering
points (public or private), it also has some poor
scaling properties and is uncomfortably reliant
upon the stability of routing.
If he's done any more thinking about the idea,
I'd love to hear about it though.
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